Blender live-action footage and Grease Pencil animation
The grease pencil works only in the 3D viewport, you can't use it on the VSE or in the Movie Clip Editor.
Here are a couple of options:
1. Use video as background for the camera. Select the camera object and, in the camera option tab, check Background image -&gt; Add image -&gt; Movie, then select your video file there.
This way you will see the video through the camera view (Numpad 0 ).

Note that the background image will not render unless you use it in the compositor, and mix your render layer using Alpha over.
Make sure that Film is set to transparent and the view transform for color management is set to Standard.

Change the resolution in the Output tab for the project so that it matches the dimensions, and the frame rate of the original video. Also remember to render at 100% of the resolution.

2. Alternatively, use the video as texture on the world.
Set the color to use the image texture, and select the video there. Set the vector coordinates to Window. and the extrapolation to Clip
In this case you need to disable the film transparent settings to make the background renderable. No need to use the compositor in this case.
To view the background enable Scene World and Scene Lights